This invention relates to a cylcopentadienyl-containing low-valent transition metal catalyst that is useful in polymerizing and co-polymerizing polar and non-polar olefin monomers, and more particularly relates to an in situ reduced Group 4 metal polymerization catalyst that is capable of forming polymers and copolymers of conjugated monomers such as methyl methacrylate (MMA) and styrene.
Catalysts based on early transition metal d0 complexes, such as Ziegler-Natta catalysts, are used extensively for coordination polymerization of nonpolar olefins such as ethylene and propylene. However, due to their highly oxophilic nature, these catalysts typically are incompatible with functionalized vinyl monomers in achieving either insertive polymerization of polar olefins or copolymerization of nonpolar olefins with polar comonomers.
Late transition metal catalysts are less oxophilic; however, most often they effect olefin dimerization or oligomerization rather than polymerization to high molecular weight polymers. Recently, Brookhart and co-workers (J. Am. Chem. Soc., 1996, 118, 267–268) reported examples of late transition metal-catalyzed insertive copolymerizations of nonpolar olefins (ethylene and propylene) with alkyl acrylates to give high molar mass (high molecular weight) polymers. However, activities become significantly lower as the polar comonomer concentration increases and polar groups are only detected at the end of polymer chain branches. Polymerization of olefins containing functional groups in a position remote from the vinyl group by early transition metal catalysts has been reported as have been very oxophilic catalysts such as lanthanocene and zirconocene to catalyze polymerization of polar monomers such as MMA or lactones through a Michael addition mechanism.
Crystalline vinyl aromatic polymers such as syndiotactic polystyrene have been produced from single-site or metallocene catalysts. EP 0 421 659 describes production of syndiotactic polystyrene using a mono-cyclopentadienyl complexed transition metal catalyst in combination with a non-coordinating anion such as a perfluoro borane or borate.
Polymers of polar vinyl monomers, such as MMA, are well known and typically are produced through a radical polymerization mechanism. Radical polymerization processes may have high polymerization activity for functionalized olefins, but usually require high pressure, produce broad molecular weight distribution resins, and do not control stereoregularity. Single-site catalysts, such as those based on a cylcopentadienyl ligand complexed with a transition metal, polymerize olefins with controllable molecular weights and stereoregularity and with narrow molecular weight distributions. However, these single-site catalysts typically do not polymerize functionalized olefins or copolymerize a functionalized with non-functionalized olefins.
Copolymerization of polar monomers with olefins using transition metal complexes is reviewed by Boffa and Novak, Chem. Rev. 2000, 100, 1479–1493, incorporated by reference herein.
Soga et al., Macromolecules, 1994, 27, 7938–7940, report formation of a syndio-rich atactic polymer of MMA using a metallocene cationic complex Cp2Zr(CH3)+B(C6F5)4− in toluene in the required presence of diethyl zinc. Also, Chen et al. J. Am. Chem. Soc., 1998, 120, 6287–6305, incorporated herein by reference, reported MMA polymerization using a binuclear {Cp2Zr(CH3)}2CH3+-type catalyst using a living group transfer process mechanism and not coordinative polymerization.
U.S. Pat. No. 5,616,748 describes formation of a neutral reduced metal titanium cyclopentadienyl complex using a lithium alkyl reducing agent, but does not describe combinations with non-coordinating anions or use as a polymerization catalyst for polar and nonpolar olefins.
Our invention relates to a catalyst system that is capable of polymerizing polar and non-polar olefins. Examples of polar/nonpolar copolymers may be stereoregular as well as containing regions of alternating monomer polar/nonpolar units.
In one aspect of this invention, a monocyclopentadienyl transition metal metallocene combined with a non-coordinating cocatalyst anion is reduced in situ with a suitable reducing agent such as zinc metal to form an active olefin polymerization catalyst system capable of polymerizing and copolymerizing both polar and nonpolar olefins.
In another aspect of this invention, a monocylcopentadienyl-containing Group 4 metal complex in combination with a non-coordinating borate anion is reduced in situ with a metallic reducing agent such as zinc to form an active olefin polymerization catalyst.
In another aspect of the invention, stereoregular copolymers of polar and nonpolar olefins are formed. In other aspect of the invention styrene and methylmethacrylate are polymerized to crystalline polymers and copolymerized to isotactic copolymers containing 10 mol % or more of methylmethacrylate monomer units. These and other aspects of this invention are described and claimed herein.
Polar copolymers generally are useful as barrier materials for packaging; have improved adhesion/paintability/wetability characteristics; have functionalization points for grafting, coating, and lamination; may be blend compatibilizers for multilayered structures; may be a replacement for halogen-coating polymers, and have improved processing and mechanical properties.